Post-synthetic Metalation on the Ionic TiO2 Surface to Enhance Metal-CO2 Interaction During Photochemical CO2 Reduction.

During the photochemical CO2 reduction reaction, CO2 adsorption on the catalyst's surface is a crucial step where the binding mode of the [metal-CO2] adduct directs the product selectivity and efficiency. Herein, an ionic TiO2 nanostructure stabilized by polyoxometalates (POM), ([POM]x@TiO2), is prepared and the sodium counter ions present on the surface to balance the POMs' charge are replaced with copper(II) ions, (Cux[POM]@TiO2). The microscopic and spectroscopic studies affirm the copper exchange without altering the TiO2 core and weak coordination of copper (II) ions to the POMs' surface. Band structure analysis suggests the photo-harvesting efficiency of the TiO2 core with the conduction band edge higher than the reduction potential of CuII/I and multi-electron CO2 reduction potentials. Photochemical CO2 reduction with Cux[POM]@TiO2 results in 30 µmol gcat.-1 CO (79%) and 8 µmol gcat-1 of CH4 (21%). Quasi-in-situ Raman study provides evidence in support of CO2 adsorption on the Cux[POM]@TiO2 surface. 13C and D2O labeling studies validates the {Cu-[CO2]-} adduct formation. Despite the photo-harvesting ability of Nax[POM]@TiO2 itself, the poor CO2 adsorption ability of sodium ions highlights the crucial role of copper ion CO2 photo-reduction. Characterization of the {M-[η2-CO2]-} species via surface tuning validates the CO2 activation and photochemical reduction pathway proposed earlier.

Structural and Electronic Factors behind the Electrochemical Stability of 3D-Metal Tungstates under Oxygen Evolution Reaction Conditions.

Transition metal tungstates (TMTs) possess a wolframite-like lattice structure and preferably form via an electrostatic interaction between a divalent transition metal cation (MII) and an oxyanion of tungsten ([WO4]2-). A unit cell of a TMT is primarily composed of two repeating units, [MO6]oh and [WO6]oh, which are held together via several M-µ2-O-W bridging links. The bond character (ionic or covalent) of this bridging unit determines the stability of the lattice and influences the electronic structure of the bulk TMT materials. Recently, TMTs have been successfully employed as an electrode material for various applications, including electrochemical water splitting. Despite the wide electrocatalytic applications of TMTs, the study of the structure-activity correlation and electronic factors responsible for in situ structural evolution to electroactive species during electrochemical reactions is still in its infancy. Herein, a series of TMTs, MIIWVIO4 (M = Mn/Fe/Co/Ni), have been prepared and employed as electrocatalysts to study the oxygen evolution reaction (OER) under alkaline conditions and to scrutinize the role of transition metals in controlling the energetics of the formation of electroactive species. Since the [WO6]oh unit is common in the TMTs considered, the variation of the central atom of the corresponding [MO6]oh unit plays an intriguing role in controlling the electronic structure and stability of the lattice under anodic potential. Under the OER conditions, a potential-dependent structural transformation of MWO4 is noticed, where MnWO4 appears to be the most labile, whereas NiWO4 is stable up to a high anodic potential of ∼1.68 V (vs RHE). Potential-dependent hydrolytic [WO4]2- dissolution to form MOx active species, traced by in situ Raman and various spectro-/microscopic analyses, can directly be related to the electronic factors of the lattice, viz., crystal field splitting energy (CFSE) of MII in [MO6]oh, formation enthalpy (ΔHf), decomposition enthalpy (ΔHd), and Madelung factor associated with the MWO4 ionic lattice. Additionally, the magnitude of the Löwdin and Bader charges on M of the M-µ2-O-W bond is directly related to the degree of ionicity or covalency in the MWO4 lattice, which indirectly influences the electronic structure and activity. The experimental results substantiated by the computational study explain the electrochemical activity of the TMTs with the help of various structural and electronic factors and bonding interactions in the lattice, which has never been realized. Therefore, the study presented here can be taken as a general guideline to correlate the reactivity to the structure of the inorganic materials.

Evolution of Mn-Bi2O3 from the Mn-doped Bi3O4Br electro(pre)catalyst during the oxygen evolution reaction.

Mn-doped Bi3O4Br has been synthesized using a solvothermal route. The undoped Bi3O4Br and Mn-Bi3O4Br materials possess orthorhombic unit cells with two distinct Bi sites forming a layered atomic arrangement. The shift in the (020) plane in the powder X-ray diffraction (PXRD) pattern confirms Mn-doping in the Bi3O4Br lattice. Elemental mapping indicated 7% Mn doping in the Bi3O4Br lattice structure. A core-level X-ray photoelectron study (XPS) indicates the presence of BiIII and MnII valence-states in Mn-Bi3O4Br. Doping with a cation (MnII) containing a different charge and ionic radius resulted in vacancy/defects in Mn-Bi3O4Br which further altered its electronic structure by reducing the indirect band gap, beneficial for electron conduction and electrocatalysis. The irreversible MnII to MnIII transformation at a potential of 1.48 V (vs. RHE) precedes the electrochemical oxygen evolution reaction (OER). The Mn-doped electrocatalyst achieved 10 mA cm-2 current density at 337 mV overpotential, while the pristine Bi3O4Br required 385 mV overpotential to reach the same activity. The pronounced OER activity of the Mn-Bi3O4Br sample over the pristine Bi3O4Br highlights the necessity of MnII doping. The superior activity of the Mn-Bi3O4Br catalyst over that of Bi3O4Br is due to a low Tafel slope, better double-layer capacitance (Cdl), and small charge-transfer resistance (Rct). The chronoamperometry (CA) study depicts long-term stability for 12 h at 20 mA cm-2. An electrolyzer fabricated as Pt(-)/(+)Mn-Bi3O4Br can deliver 10 mA cm-2 at a cell potential of 2.05 V. The post-CA-OER analyses of the anode confirmed the leaching of [Br-] followed by in situ formation of Mn-doped Bi2O3 as the electrocatalytically active species. Herein, an ultra-low Mn-doping into Bi3O4Br leads to an improvement in the electrocatalytic performance of the inactive Bi3O4Br material.

Elevated temperature-driven coordinative reconstruction of an unsaturated single-Ni-atom structure with low valency on a polymer-derived matrix for the electrolytic oxygen evolution reaction.

A high-temperature pyrolysis-controlled coordination reconstruction resulted in a single-Ni-atom structure with a Ni-Nx-C structural unit (x = N atom coordinated to Ni). Pyrolysis of Ni-phen@ZIF-8-RF at 700 °C resulted in NiNP-NC-700 with predominantly Ni nanoparticles. Upon elevating the pyrolysis temperature from 700 to 900 °C, a coordination reconstruction offers Ni-Nx atomic sites in NiSA-NC-900. A combined investigation with X-ray absorption spectroscopy, X-ray photoelectron spectroscopy, and soft X-ray L3-edge spectroscopy suggests the stabilization of low-valent Niδ+ (0 < δ < 2) in the Ni-N-C structural units. The oxygen evolution reaction (OER) is a key process during water splitting in fuel cells. However, OER is a thermodynamically uphill reaction with multi-step proton-coupled electron transfer and sluggish kinetics, due to which there is a need for a catalyst that can lower the OER overpotentials. The adsorption energy of a multi-step reaction on a single metal atom with coordination unsaturation tunes the adsorption of each oxygenated intermediate. The promising OER activity of the NiSA-NC-900/NF anode on nickel foam was followed by the overall water splitting (OWS) using using NiSA-NC-900/NF as anode and Pt coil as the cathodic counterpart, wherein a cell potential of 1.75 V at 10 mA cm-2 was achieved. The cell potential recorded with Pt(-)/(+)NiSA-NC-900/NF was much lower than that obtained for other cells, i.e., Pt(-)/NF and NF(-)/(+)NF, which enhances the potentials of low-valent NiSAs for insightful understanding of the OER. At a constant applied potential of 1.61 V (vs. RHE) for 12 h, an small increase in current for initial 0.6 h followed by a constant current depicts the fair stability of catalyst for 12 h. Our results offer an insightful angle into the OER with a coordinatively reconstructed single-Ni-atom structure at lower valency (<+2).

Surface Structure to Tailor the Electrochemical Behavior of Mixed-Valence Copper Sulfides during Water Electrolysis.

The semiconducting behavior of mixed-valence copper sulfides arises from the pronounced covalency of Cu-S bonds and the exchange coupling between CuI and CuII centers. Although electrocatalytic study with digenite Cu9S5 and covellite CuS has been performed earlier, detailed redox chemistry and its interpretation through lattice structure analysis have never been realized. Herein, nanostructured Cu9S5 and CuS are prepared and used as electrode materials to study their electrochemistry. Powder X-ray diffraction (PXRD) and microscopic studies have found the exposed surface of Cu9S5 to be d(0015) and d(002) for CuS. Tetrahedral (Td) CuII, distorted octahedral (Oh) CuII, and trigonal planar (Tp) CuI sites form the d(0015) surface of Cu9S5, while the (002) surface of CuS consists of only Td CuII. The distribution of CuI and CuII sites in the lattice, predicted by PXRD, can further be validated through core-level Cu 2p X-ray photoelectron spectroscopy (XPS). The difference in the electrochemical response of Cu9S5 and CuS arises predominantly from the different copper sites present in the exposed surfaces and their redox states. In situ Raman spectra recorded during cyclic voltammetric study indicates that Cu9S5 is more electrochemically labile compared to CuS and transforms rapidly to CuO/Cu2O. Contact-angle and BET analyses imply that a high-surface-energy and macroporous Cu9S5 surface favors the electrolyte diffusion, which leads to a pronounced redox response. Post-chronoamperometric (CA) characterizations identify the potential-dependent structural transformation of Cu9S5 and CuS to CuO/Cu2O/Cu(OH)2 electroactive species. The performance of the in situ formed copper-oxides towards electrocatalytic water-splitting is superior compared to the pristine copper sulfides. In this study, the redox chemistry of the Cu9S5/CuS has been correlated to the atomic arrangements and coordination geometry of the surface exposed sites. The structure-activity correlation provides in-depth knowledge of how to interpret the electrochemistry of metal sulfides and their in situ potential-driven surface/bulk transformation pathway to evolve the active phase.

Elucidating the Role of Atomically Dilute Copper Centers Impregnating a Phosphamide Polymer for the Preferential Hydrogen Evolution Reaction over CO2 Reduction.

Organic polymers have attracted considerable interest in designing a multifunctional electrocatalyst. However, the inferior electro-conductivity of such metal-free polymers is often regarded as a shortcoming. Herein, a nitrogen- and phosphorus-rich polymer with phosphamide functionality (PAP) in the repeating unit has been synthesized from diaminopyridine (DAP) and phenylphosphonic dichloride (PPDC) precursors. The presence of phosphamide oxygen and pyridine nitrogen in the repeating unit of PAP leads to the coordination of the CuII ion and the incorporation of 3.29 wt % in the polymer matrix (Cu30@PAP) when copper salt is used to impregnate the polymer. Combined with a spectroscopic, microscopic, and DFT study, the coordination and geometry of copper in the PAP matrix has been established to be a distorted square planar CuII in a N2O2 ligand environment where phosphamide oxygen and pyridine nitrogen of the PAP coordinate to the metal center. The copper incorporation in the PAP modulates its electrocatalytic activity. On the glassy carbon electrode, PAP shows inferior activity toward the hydrogen evolution reaction (HER) in 0.5 M H2SO4 while 3 wt % copper incorporation (Cu30@PAP) significantly improves the HER performance with an overpotential of 114 mV at 10 mA cm-2. The notable electrochemical activity with Cu30@PAP occurs due to the impregnation of Cu(II) in PAP, improved electro-kinetics, and better charge transfer resistance (Rct). When changing the electrolyte from H2SO4 to CO2-saturated bicarbonate solution at nearly neutral pH, PAP shows HER as the dominant pathway along with the partial reduction of CO2 to formate. Moreover, the use of Cu30@PAP as an electrolcatalyst could not alter the predominant HER path, and only 20% Faradaic efficiency for the CO2 reduced products has been achieved. Post-chronoamperometric characterization of the recovered catalyst suggests an unaltered valence state of the copper ion and the intact chemical structure of PAP. DFT studies unraveled that the copper sites of Cu30@PAP promote water adsorption while phosphamide-NH of the PAP can weakly hold the CO2 adduct via a hydrogen bonding interaction. A detailed calculation has pointed out that the tetra-coordinated copper centers present in the PAP frame are the reactive sites and that the formation of the [CuI-H] intermediate is the rate-limiting step for both HER and its competitive side reaction, i.e., CO2 reduction to formate or CO formation. The high proton concentration in the electrolyte of pH < 7 leads to HER as the predominant pathway. This combined experimental and theoretical study has highlighted the crucial role of copper sites in electrocatalysis, emphasizing the plausible reason for electrocatalytic selectivity.

Mechanistic Insights into the Appel Reaction Mediated by a Poly-Phosphamide Material as a Heterogeneous Catalyst.

Due to notable thermochemical stability, polyphosphamides are often regarded as flame retardants, while molecular phosphamides can serve as versatile Lewis base to catalyze diverse organic transformations. Being chemically analogous to phosphine oxide, phosphamide can also be considered as a mediator for the phosphine-mediated reaction. Herein, an amorphous polymeric material consisting of phosphamide (-NH-P(O)) in the repeating unit (POP) has been prepared via condensation of tris(2-aminoethyl)amine (TREN) and phenyl phosphinic dichloride (PPDC). The POP is isolated as a metal-free and pure organic material which is made of a strong covalent bond and the phosphamide unit is deployed in the organic framework. The presence of phosphamide in the repeating unit of the isolated amorphous POP material can be confirmed by 31P CPMAS NMR, FTIR, and Raman studies. The core-level N 2p and P 2p X-ray photoelectron spectra are in accordance with the presence of tertiary amine nitrogen attached to carbon and secondary amine nitrogen attached to phosphorus. Elemental analyses have depicted approximately 19.7% of phosphorus content in the material, which is being utilized to study the catalytic Appel reaction with 76% conversion of alcohol to a corresponding halide and TON of 462. Quasi in situ Raman study has identified that amino phosphine formed via in situ reduction of the phosphamide unit of the POP catalyzes the halogenation of primary and secondary alcohols with wide substrate scope and functional group tolerance. Kinetic studies have established a first-order dependence with respect to alcohol, while deuterium labeling experiments emphasize that the deprotonation of alcohol is the rate-limiting step. High thermal stability of the material, scope of easy catalyst recyclability, and a cumulative TON of 1386 have led the POP as an emerging pure organic material to be explored further for other phosphine-mediated organocatalysis.

Ease of Electrochemical Arsenate Dissolution from FeAsO4 Microparticles during Alkaline Oxygen Evolution Reaction.

Transition metal-based ABO4-type materials have now been paid significant attention due to their excellent electrochemical activity. However, a detailed study to understand the active species and its electro-evolution pathway is not traditionally performed. Herein, FeAsO4, a bimetallic ABO4-type oxide, has been prepared solvothermally. In-depth microscopic and spectroscopic studies showed that the as-synthesized cocoon-like FeAsO4 microparticles consist of several small individual nanocrystals with a mixture of monoclinic and triclinic phases. While depositing FeAsO4 on three-dimensional nickel foam (NF), it can show oxygen evolution reaction (OER) in a moderate operating potential. During the electrochemical activation of the FeAsO4/NF anode through cyclic voltammetric (CV) cycles prior to the OER study, an exponential increment in the current density (j) was observed. An ex situ Raman study with the electrode along with field emission scanning electron microscopy imaging showed that the pronounced OER activity with increasing number of CV cycles is associated with a rigorous morphological and chemical change, which is followed by [AsO4]3- leaching from FeAsO4. A chronoamperometric study and subsequent spectro- and microscopic analyses of the isolated sample from the electrode show an amorphous γ-FeO(OH) formation at the constant potential condition. The in situ formation of FeO(OH)ED (ED indicates electrochemically derived) shows better activity compared to pristine FeAsO4 and independently prepared FeO(OH). Tafel, impedance spectroscopic study, and determination of electrochemical surface area have inferred that the in situ formed FeO(OH)ED shows better electro-kinetics and possesses higher surface active sites compared to its parent FeAsO4. In this study, the electrochemical activity of FeAsO4 has been correlated with its structural integrity and unravels its electro-activation pathway by characterizing the active species for OER.

Immobilization of Phosphamide on the TiO2 Surface for Heterogeneous Phase Catalytic Appel Reaction.

Global consumption of triphenylphosphine (Ph3P) for phosphorus-mediated organic synthesis and production of the dead-end triphenylphosphine oxide (Ph3PO) waste is exceptionally high. Recycling Ph3PO and/or use of it as a reaction mediator gained significant attention. On the other end, phosphamides, traditionally used as a flame redundant, are stable analogues to Ph3PO. Herein, via a low temperature condensation reaction of methyl 4-(aminomethyl)benzoate (AMB) and diphenyl phosphinic chloride (DPPC), methyl 4-((N,N-diphenylphosphinamido)methyl)benzoate (1) has been synthesized and hydrolysis of the ester functional group of 1 leads to a phosphamide with a carboxylate terminal, 4-((N,N-diphenylphosphinamido)methyl)benzoic acid (2). The presence of phosphamide functionality (NH─P═O) in 2 can be confirmed by its characteristic Raman vibration at 999 cm-1 with expected P-N and P═O bonds distances from the single-crystal X-ray structure. In-situ hydrolysis of [Ti(OiPr)4] in the presence of 2 followed by hydrothermal heating results in immobilization of 2 on a ca. 5 nm TiO2 surface (2@TiO2). The covalent attachment of 2 via coordination through the carboxylate terminal to the TiO2 nanocrystal's surface has been established via multiple spectroscopic and microscopic studies. 2@TiO2 is further used as the heterogeneous mediator for the catalytic Appel reaction, halogenation of alcohol (typically mediated by phosphine), with a fair catalytic conversion and a recorded TON up to 31. The major advantage of the heterogeneous approach studied herein is the recovery of used 2@TiO2 from the reaction mixture via centrifugation only leaving the organic product in the supernatant, which is limiting in Ph3P-mediated homogeneous catalysis. Time-resolved Raman spectroscopy confirms amino phosphine as the active species formed in-situ during the catalytic Appel reaction. Post-catalytic characterization of the material recovered after catalysis from the reaction mixture confirms the chemical integrity and that can further be utilized for another two catalytic runs. The developed reaction scheme showcases the use of a phosphamide as a reactive analogue to Ph3PO for an organic reaction in a heterogeneous approach, and the same strategy can be explored further as a general scheme for other phosphorus-mediated reactions.

Redox-active Sn(II) to lead to SnFe2O4 spinel as a bi-functional water splitting catalyst.

Despite several reports on metal ferrites for water splitting studies, SnFe2O4 is a rarely explored spinel oxide. Herein, solvothermally prepared ca. 5 nm SnFe2O4 nanoparticles deposited on nickel foam (NF) behaves as a bi-functional electrocatalyst. In alkaline pH, the SnFe2O4/NF electrode exhibits oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) at moderate overpotentials and shows a fair chronoamperometric stability. Detailed study indicates that iron sites of the spinel are preferably active for the OER while the SnII sites not only enhance the electrical conductivity of the material but also favor the HER.

Integrating Electrochemical CO2 Reduction on α-NiS with the Water or Organic Oxidations by Its Electro-Oxidized NiO(OH) Counterpart to an Artificial Photosynthetic Scheme.

Efficient hydrogen production, biomass up-conversion, and CO2-to-fuel generation are the key challenges of the present decade. Electrocatalysis in aqueous electrolytes by choosing suitable transition-metal-based electrode materials remains the green approach for the trio of sustainable developments. Given that, finding electrode materials with multifunctional capability would be beneficial. Herein, the nanocrystalline α-NiS, synthesized solvothermally, has been chosen as an electrode material. As the first step to construct an electrolyzer, α-NiS deposited on conducting nickel foam (NF) has been used as an anode, and under the anodic potential, the α-NiS particles have lost sulfides to the electrolyte and transform to amorphous electro-derived NiO(OH) (NiO(OH)ED), confirmed by different spectroscopic and microscopic studies. In situ transformation of α-NiS to amorphous NiO(OH)ED results in an enhancement of the electrochemical surface area and not only becomes active toward oxygen evolution reaction (OER) at a moderate overpotential of 264 mV (at 20 mA cm-2) but also can convert a series of biomass-derived organic compounds, namely, 2-hydroxymethylfurfural (HMF), 2-furfural (FF), ethylene glycol (EG), and glycerol (Gly), to industrially relevant feedstocks with a high (∼96%) Faradaic efficiency. During these organic oxidations, NiO(OH)ED/NF participate in the multiple-electron oxidation process (up to 8e-) including C-C bond cleavages of EG and Gly. During the cathodic performance of the α-NiS/NF, the structural integrity has been retained and the unaltered α-NiS/NF electrode remains more effective cathode for alkaline hydrogen evolution reaction (HER) and CO2 reduction (CO2R) compared to its in situ-derived NiO(OH)ED/NF. α-NiS/NF can reduce the CO2 predominantly to CO even at a higher potential like -0.8 V (vs RHE). The fabricated cell with α-NiS and its electro-oxidized NiO(OH)ED counterpart, α-NiS/NF(-)/(+)NiO(OH)ED/NF, is able to show an artificial photosynthetic scheme in which the NiO(OH)ED/NF anode oxidizes water to O2 and the α-NiS cathode reduces CO2 majorly to CO in a moderate cell potential. In this study, α-NiS has been utilized as a single electrode material to perform multiple sustainable transformations.

Ionic γ-FeO(OH) Nanocrystal Stabilized by Small Isopolymolybdate Clusters as Reactive Core for Water Oxidation.

At near neutral to basic pH, hydrolysis-induced aggregation to insoluble bulk iron-oxide is often regarded as the pitfalls of molecular iron clusters. Iron-oxide nanocrystals are encouragingly active over the molecular clusters and/or bulk oxides albeit, stabilizing such nanostructures in aqueous pH and under turnover condition remain a perdurable challenge. Herein, an Anderson-type [Mo7 O24 ]6- isopolyanion, a small (dimension ca. 0.85 nm) isolable polyoxometalate (POM) possessing only {31} atoms, has been introduced for the first time as a covalent linker to stabilize an infinitely stable and aqueous-soluble γ-FeO(OH) nanocore. During the hydrothermal isolation of the material, a partial dissociation of the parent [Mo7 O24 ]6- may lead to the in situ generation of few analogous [Mox Oy ]n- clusters, proved by Raman study, which can also participate in stabilizing the γ-FeO(OH) nanocore, Mox Oy @FeO(OH). However, due to high ionic charge on {Mo=O} terminals of the [Mox Oy ]n- , they are covalently linked via MoVI -µ2 O-FeIII bridging to γ-FeO(OH) core in Mox Oy @FeO(OH), established by numerous spectroscopic and microscopic evidence. Such bonding mode is more likely as precedent from the coordination motif documented in the transition metal clusters stabilized by this POM. The γ-FeO(OH) nanocore of Mox Oy @FeO(OH) behaves as potent active center for electrochemical water oxidation with a overpotential, 263 mV @ 10 mA cm-2 , lower than that observed for bare γ-FeO(OH). Despite of some molybdenum dissolution from the POM ligands to the electrolyte, residual anionic POM fragments covalently bound to the OER active γ-FeO(OH) core of the Mox Oy @FeO(OH) makes the surface predominantly ionic that results in an ordered electrical double layer to promote a better charge transport across the electrode-electrolyte junction, less likely in bulk γ-FeO(OH).

γ-FeO(OH) with multiple surface terminations: Intrinsically active for the electrocatalytic oxygen evolution reaction.

Due to poor conductivity, the electrocatalytic performance of independently prepared iron oxy-hydroxide (FeO(OH)) is inferior whereas FeO(OH) derived in situ from the iron based electro(pre)catalyst shows superior performance in the oxygen evolution reaction (OER). Use of mixed phase FeO(OH) and/or incorporation of CoII/NiII metal into the FeO(OH) structure has also been demonstrated as a convenient approach to achieve high OER activity. Nevertheless, preparation of phase-pure, albeit active FeO(OH) material with fair electrochemical performance remains a perdurable challenge. Moreover, the role of the crystalline phase and its surface structure in controlling the OER activity is still unclear. Herein, a simple synthetic protocol has been developed to prepare a series of phase-pure α-FeO(OH) (goethite) and γ-FeO(OH) (lepidocrocite) materials. By changing the reaction conditions such as iron salt and reaction temperature, the crystallinity as well as the phase of the oxy-hydroxide material have been varied. The isolated α- and γ-FeO(OH) materials with different crystallinity were thereafter deposited on nickel foam (NF) for alkaline OER study. The recorded overpotential value at 10 mA cm-2 has been found to be dependent on the phase and crystallinity of the FeO(OH) materials. The partially crystalline γ-FeO(OH) isolated at room temperature (γ-FeO(OH)@RT) turns out to be the most active with a lowest overpotential of 260 mV at 10 mA cm-2 and a long term stability of 12 h. The γ-FeO(OH)@RT/NF anode can furnish high current densities like 50-100 mA cm-2 which makes this anode distinct from the previously reported FeO(OH) materials. Detailed electrochemical study suggested that the fair activity of the γ-FeO(OH)@RT arises due to a facile electrokinetics as evident from the small Tafel slope and charge transfer resistance (Rct value from the Nyquist plot). Owing to the superior activity of the γ-FeO(OH)@RT/NF, the anode can further be incorporated into an overall water splitting electrolyzer that can operate at a cell potential of 1.68 V. The microscopic characterization provides concrete evidence in support of the polycrystallinity of the γ-FeO(OH)@RT. The superior activity of the γ-FeO(OH)@RT perhaps can be correlated to its polycrystalline nature with more defect edges, the presence of a large exposed surface and random atomic arrangements. The highest degree of multiple surface active terminals (-O, -OH and -Fe) available in this polycrystalline γ-FeO(OH) perhaps makes the catalyst more active compared to the crystalline FeO(OH) analogue with a limited number of surface terminals. From a comparative study with a series of FeO(OH) materials, this work highlights a direct relationship between the surface functionality and the electrochemical activity of the FeO(OH) material.

Intrinsic Lability of NiMoO4 to Excel the Oxygen Evolution Reaction.

Nickel-based bimetallic oxides such as NiMoO4 and NiWO4, when deposited on the electrode substrate, show remarkable activity toward the electrocatalytic oxygen evolution reaction (OER). The stability of such nanostructures is nevertheless speculative, and catalytically active species have been less explored. Herein, NiMoO4 nanorods and NiWO4 nanoparticles are prepared via a solvothermal route and deposited on nickel foam (NF) (NiMoO4/NF and NiWO4/NF). After ensuring the chemical and structural integrity of the catalysts on electrodes, an OER study has been performed in the alkaline medium. After a few cyclic voltammetry (CV) cycles within the potential window of 1.0-1.9 V (vs reversible hydrogen electrode (RHE)), ex situ Raman analysis of the electrodes infers the formation of NiO(OH)ED (ED: electrochemically derived) from NiMoO4 precatalyst, while NiWO4 remains stable. A controlled study, stirring of NiMoO4/NF in 1 M KOH without applied potential, confirms that NiMoO4 hydrolyzes to the isolable NiO, which under a potential bias converts into NiO(OH)ED. Perhaps the more ionic character of the Ni-O-Mo bond in the NiMoO4 compared to the Ni-O-W bond in NiWO4 causes the transformation of NiMoO4 into NiO(OH)ED. A comparison of the OER performance of electrochemically derived NiO(OH)ED, NiWO4, ex-situ-prepared Ni(OH)2, and NiO(OH) confirmed that in-situ-prepared NiO(OH)ED remained superior with a substantial potential of 238 (±6) mV at 20 mA cm-2. The notable electrochemical performance of NiO(OH)ED can be attributed to its low Tafel slope value (26 mV dec-1), high double-layer capacitance (Cdl, 1.21 mF cm-2), and a low charge-transfer resistance (Rct, 1.76 Ω). The NiO(OH)ED/NF can further be fabricated as a durable OER anode to deliver a high current density of 25-100 mA cm-2. Post-characterization of the anode proves the structural integrity of NiO(OH)ED even after 12 h of chronoamperometry at 1.595 V (vs reversible hydrogen electrode (RHE)). The NiO(OH)ED/NF can be a compatible anode to construct an overall water splitting (OWS) electrolyzer that can operate at a cell potential of 1.64 V to reach a current density of 10 mA cm-2. Similar to that on NF, NiMoO4 deposited on iron foam (IF) and carbon cloth (CC) also electrochemically converts into NiO(OH) to perform a similar OER activity. This work understandably demonstrates monoclinic NiMoO4 to be an inherently unstable electro(pre)catalyst, and its structural evolution to polycrystalline NiO(OH)ED succeeding the NiO phase is intrinsic to its superior activity.

Dioxygen Activation and Mandelate Decarboxylation by Iron(II) Complexes of N4 Ligands: Evidence for Dioxygen-Derived Intermediates from Cobalt Analogues.

The isolation, characterization, and dioxygen reactivity of monomeric [(TPA)MII(mandelate)]+ (M = Fe, 1; Co, 3) and dimeric [(BPMEN)2MII2(µ-mandelate)2]2+ (M = Fe, 2; Co, 4) (TPA = tris(2-pyridylmethyl)amine and BPMEN = N1,N2-dimethyl-N1,N2-bis(pyridin-2-yl-methyl)ethane-1,2-diamine) complexes are reported. The iron(II)- and cobalt(II)-mandelate complexes react with dioxygen to afford benzaldehyde and benzoic acid in a 1:1 ratio. In the reactions, one oxygen atom from dioxygen is incorporated into benzoic acid, but benzaldehyde does not derive any oxygen atom from dioxygen. While no O2-derived intermediate is observed with the iron(II)-mandelate complexes, the analogous cobalt(II) complexes react with dioxygen at a low temperature (-80 °C) to generate the corresponding cobalt(III)-superoxo species (S), a key intermediate implicated in the initiation of mandelate decarboxylation. At -20 °C, the cobalt(II)-mandelate complexes bind dioxygen reversibly leading to the formation of µ-1,2-peroxo-dicobalt(III)-mandelate species (P). The geometric and electronic structures of the O2-derived intermediates (S and P) have been established by computational studies. The intermediates S and P upon treatment with a protic acid undergo decarboxylation to afford benzaldehyde (50%) with a concomitant formation of the corresponding µ-1,2-peroxo-µ-mandelate-dicobalt(III) (P1) species. The crystal structure of a peroxide species isolated from the cobalt(II)-carboxylate complex [(TPA)CoII(MPA)]+ (5) (MPA = 2-methoxyphenylacetate) supports the composition of P1. The observations of the dioxygen-derived intermediates from cobalt complexes and their electronic structure analyses not only provide information about the nature of active species involved in the decarboxylation of mandelate but also shed light on the mechanistic pathway of two-electron versus four-electron reduction of dioxygen.

Electrocatalytic water oxidation performance in an extended porous organic framework with a covalent alliance of distinct Ru sites.

The rational synthesis of durable, earth-abundant efficient electrocatalysts for the oxygen evolution reaction (OER) from water is one of the most important routes for storing renewable energy and minimizing fossil fuel combustion. The prime hurdles for effectively utilizing commercial RuO2 as (OER) electrocatalysts are its very low stability, catalyst deactivation, and high cost. In this work, we explored a Ru-integrated porous organic polymer (Ru@Bpy-POP) by a facile one-pot Friedel-Crafts alkylation strategy between redox-active (Ru(demob)3Cl2) and a carbazole unit, which is composed of unique features including an extended framework unit, isolated active sites, and tunable electrode kinetics. Ru@Bpy-POP can serve as a bridge between a Metal-Organic Framework (MOF) and POP-based catalytic systems with a balanced combination of covalent bonds (structural stability) and open metal sites (single site catalysis). Ru@Bpy-POP, deposited on a three-dimensional nickel foam electrode support, exhibits a promising electrocatalytic OER activity with an ultra-low ruthenium loading compared to a benchmark RuO2 catalyst, providing an overpotential of about 270 mV to reach 10 mA cm-2 in an alkaline medium. Moreover, a high current density of 248 mA cm-2 was achieved for the Ru@Bpy-POP catalyst at only 1.6 V (vs. RHE), which is much higher than 91 mA cm-2 for commercial RuO2. The robust, albeit highly conjugated, POP framework not only triggered facile electro-kinetics but also suppressed aggregation and metallic corrosion during electrolysis. In particular, the benefits of covalent integration of distinct Ru sites into the framework can modulate intermediate adsorption and charge density, which contributes to its exceptional OER activity. All of the critical steps involved in OER are complemented by Density Functional Theory (DFT) calculations, which suggest that electrocatalytic water oxidation proceeds from a closed-shell configuration to open-shell electronic configurations with high-spin states. These open-shell configurations are more stable than their closed-shell counterparts by 1 eV, improving the overall catalytic activity.

Electrochemically Derived Crystalline CuO from Covellite CuS Nanoplates: A Multifunctional Anode Material.

In the present era, electrochemical water splitting has been showcased as a reliable solution for alternative and sustainable energy development. The development of a cheap, albeit active, catalyst to split water at a substantial overpotential with long durability is a perdurable challenge. Moreover, understanding the nature of surface-active species under electrochemical conditions remains fundamentally important. A facile hydrothermal approach is herein adapted to prepare covellite (hexagonal) phase CuS nanoplates. In the covellite CuS lattice, copper is present in a mixed-valent state, supported by two different binding energy values (932.10 eV for CuI and 933.65 eV for CuII) found in X-ray photoelectron spectroscopy analysis, and adopted two different geometries, that is, trigonal planar preferably for CuI and tetrahedral preferably for CuII. The as-synthesized covellite CuS behaves as an efficient electro(pre)catalyst for alkaline water oxidation while deposited on a glassy carbon and nickel foam (NF) electrodes. Under cyclic voltammetry cycles, covellite CuS electrochemically and irreversibly oxidized to CuO, indicated by a redox feature at 1.2 V (vs the reversible hydrogen electrode) and an ex situ Raman study. Electrochemically activated covellite CuS to the CuO phase (termed as CuSEA) behaves as a pure copper-based catalyst showing an overpotential (η) of only 349 (±5) mV at a current density of 20 mA cm-2, and the TOF value obtained at η349 (at 349 mV) is 1.1 × 10-3 s-1. A low Rct of 5.90 Ω and a moderate Tafel slope of 82 mV dec-1 confirm the fair activity of the CuSEA catalyst compared to the CuS precatalyst, reference CuO, and other reported copper catalysts. Notably, the CuSEA/NF anode can deliver a constant current of ca. 15 mA cm-2 over a period of 10 h and even a high current density of 100 mA cm-2 for 1 h. Post-oxygen evolution reaction (OER)-chronoamperometric characterization of the anode via several spectroscopic and microscopic tools firmly establishes the formation of crystalline CuO as the active material along with some amorphous Cu(OH)2 via bulk reconstruction of the covellite CuS under electrochemical conditions. Given the promising OER activity, the CuSEA/NF anode can be fabricated as a water electrolyzer, Pt(-)//(+)CuSEA/NF, that delivers a j of 10 mA cm-2 at a cell potential of 1.58 V. The same electrolyzer can further be used for electrochemical transformation of organic feedstocks like ethanol, furfural, and 5-hydroxymethylfurfural to their respective acids. The present study showcases that a highly active CuO/Cu(OH)2 heterostructure can be constructed in situ on NF from the covellite CuS nanoplate, which is not only a superior pure copper-based electrocatalyst active for OER and overall water splitting but also for the electro-oxidation of industrial feedstocks.

Selective oxygenation of C-H and CC bonds with H2O2 by high-spin cobalt(II)-carboxylate complexes.

Four cobalt(II)-carboxylate complexes [(6-Me3-TPA)CoII(benzoate)](BPh4) (1), [(6-Me3-TPA)CoII(benzilate)](ClO4) (2), [(6-Me3-TPA)CoII(mandelate)](BPh4) (3), and [(6-Me3-TPA)CoII(MPA)](BPh4) (4) (HMPA = 2-methoxy-2-phenylacetic acid) of the 6-Me3-TPA (tris((6-methylpyridin-2-yl)methyl)amine) ligand were isolated to investigate their ability in H2O2-dependent selective oxygenation of C-H and CC bonds. All six-coordinate complexes contain a high-spin cobalt(II) center. While the cobalt(II) complexes are inert toward dioxygen, each of these complexes reacts readily with hydrogen peroxide to form a diamagnetic cobalt(III) species, which decays with time leading to the oxidation of the methyl groups on the pyridine rings of the supporting ligand. Intramolecular ligand oxidation by the cobalt-based oxidant is partially inhibited in the presence of external substrates, and the substrates are converted to their corresponding oxidized products. Kinetic studies and labelling experiments indicate the involvement of a metal-based oxidant in affecting the chemo- and stereo-selective catalytic oxygenation of aliphatic C-H bonds and epoxidation of alkenes. An electrophilic cobalt-oxygen species that exhibits a kinetic isotope effect (KIE) value of 5.3 in toluene oxidation by 1 is proposed as the active oxidant. Among the complexes, the cobalt(II)-benzoate (1) and cobalt(II)-MPA (4) complexes display better catalytic activity compared to their α-hydroxy analogues (2 and 3). Catalytic studies with the cobalt(II)-acetonitrile complex [(6-Me3-TPA)CoII(CH3CN)2](ClO4)2 (5) in the presence and absence of externally added benzoate support the role of the carboxylate co-ligand in oxidation reactions. The proposed catalytic reaction involves a carboxylate-bridged dicobalt complex in the activation of H2O2 followed by the oxidation of substrates by a metal-based oxidant.

Combination of Highly Efficient Electrocatalytic Water Oxidation with Selective Oxygenation of Organic Substrates using Manganese Borophosphates.

One of the key catalytic reactions for life on earth, the oxidation of water to molecular oxygen, occurs in the oxygen-evolving complex of the photosystem II (PSII) mediated by a manganese-containing cluster. Considerable efforts in this research area embrace the development of efficient artificial manganese-based catalysts for the oxygen evolution reaction (OER). Using artificial OER catalysts for selective oxygenation of organic substrates to produce value-added chemicals is a worthwhile objective. However, unsatisfying catalytic performance and poor stability have been a fundamental bottleneck in the field of artificial PSII analogs. Herein, for the first time, a manganese-based anode material is developed and paired up for combining electrocatalytic water oxidation and selective oxygenations of organics delivering the highest efficiency reported to date. This can be achieved by employing helical manganese borophosphates, representing a new class of materials. The uniquely high catalytic activity and durability (over 5 months) of the latter precursors in alkaline media are attributed to its unexpected surface transformation into an amorphous MnOx phase with a birnessite-like short-range order and surface-stabilized MnIII sites under extended electrical bias, as unequivocally demonstrated by a combination of in situ Raman and quasi in situ X-ray absorption spectroscopy as well as ex situ methods.

Tuning the stereoelectronic factors of iron(II)-2-aminophenolate complexes for the reaction with dioxygen: oxygenolytic C-C bond cleavage vs. oxidation of complex.

Oxidative C-C bond cleavage of 2-aminophenols mediated by transition metals and dioxygen is a topic of great interest. While the oxygenolytic C-C bond cleavage reaction relies on the inherent redox non-innocent property of 2-aminophenols, the metal complexes of 2-aminophenolates often undergo 1e-/2e- oxidation events (metal or ligand oxidation), instead of the direct addition of O2 for subsequent C-C bond cleavage. In this work, we report the isolation, characterization and dioxygen reactivity of a series of ternary iron(ii)-2-aminophenolate complexes [(TpPh,Me)FeII(X)], where X = 2-amino-4-tert-butylphenolate (4-tBu-HAP) (1); X = 2-amino-4,6-di-tert-butylphenolate (4,6-di-tBu-HAP) (2); X = 2-amino-4-nitrophenolate (4-NO2-HAP)(3); and X = 2-anilino-4,6-di-tert-butylphenolate (NH-Ph-4,6-di-tBu-HAP) (4) supported by a facial tridentate nitrogen donor ligand (TpPh,Me = hydrotris(3-phenyl-5-methylpyrazol-1-yl)borate). Another facial N3 ligand (TpPh2 = hydrotris(3,5-diphenyl-pyrazol-1-yl)borate) has been used to isolate an iron(ii)-2-anilino-4,6-di-tert-butylphenolate complex (5) for comparison. Both [(TpPh,Me)FeII(4-tBu-HAP)] (1) and [(TpPh,Me)FeII(4,6-di-tBu-HAP)] (2) undergo regioselective oxidative aromatic ring fission reaction of the coordinated 2-aminophenols to the corresponding 2-picolinic acids in the reaction with dioxygen. In contrast, complex [(TpPh,Me)FeII(4-NO2-HAP)] (3) displays metal based oxidation to form an iron(iii)-2-amidophenolate complex. Complexes [(TpPh,Me)FeII(NH-Ph-4,6-di-tBu-HAP)] (4) and [(TpPh2)FeII(NH-Ph-4,6-di-tBu-HAP)] (5) react with dioxygen to undergo 2e- oxidation with the formation of the corresponding iron(iii)-2-iminobenzosemiquinonato radical species implicating the importance of the -NH2 group in directing the C-C bond cleavage reactivity of 2-aminophenols. The systematic study presented in this work unravels the effect of the electronic and structural properties of the redox non-innocent 2-aminophenolate ring and the supporting ligand on the C-C bond cleavage reactivity vs. the metal/ligand oxidation of the complexes. The study further reveals that proper modulation of the stereoelectronic factors enables us to design a well synchronised proton transfer (PT) and dioxygen binding events for complexes 1 and 2 that mimic the structure and function of the nonheme enzyme 2-aminophenol-1,6-dioxygenase (APD).